Platelets aggregate due to injury of vascular endothelial cells and other various factors. When a coronary vessel, a cerebral vessel or a peripheral vessel is occluded by this platelet thrombus, myocardial infarction, cerebral infarction or chronic artery obstruction is caused, respectively. Examples of thrombotic diseases following such an activation (stimulation of aggregation) of the platelet include arterial sclerosis, ischemic cerebral infarction, ischemic cardiac diseases including myocardial infarction and angina, chronic artery obstruction and venous thrombosis.
Medicaments with the action to inhibit the platelet aggregation are used as preventive drugs for ischemic diseases carrying with the above various diseases as complications, and as preventive drugs for pathological conditions subsequent to hypertension, pulmonary hypertension, cerebral infarction, pulmonary infarction and subarachnoid hemorrhage. In addition, the above medicaments are used for prevention of thrombus formation upon Percutaneous transluminal coronary angioplasty (PTCA) and stent placing, and also used as preventive agents of restenosis after placing the stent by enfolding such a medicament having the action to inhibit the platelet aggregation by, applying it on or embedding it in the stent itself.
Meanwhile, a mosquito punctures skin with its sharp oral needle which reaches the peripheral blood vessel when it sucks blood, and frequently repeats a puncture-in and -out behavior referred to as probing in order to find out the peripheral blood vessel. It is believed that the mosquito simultaneously secrets saliva containing a substance to facilitate vasodilation to make the detection of the blood vessel easy. Due to the above probing, the peripheral blood vessel is often damaged to become congested. Generally, when the blood vessel is injured, collagen in tissue under vascular endothelium is exposed, adenosine diphosphate (ADP) is released from broken cells, and coagulation factors are activated to form thrombin. Thrombin strongly activates the platelet to induce platelet adhesion, the platelet aggregation and granule release, and eventually forms a firm thrombus by blood coagulation with fibrin formation (hemostasis mechanism). It has been known that the saliva of the mosquito contains the substance which inhibits such a hemostasis mechanism (see Non-patent literature 1).
A salivary gland protein in the mosquito, which has been studied in the most detail is apyrase. This enzyme is a platelet aggregation inhibiting substance which was identified in the saliva in Aedes aegypti for the first time. Apyrase inhibits the platelet aggregation resulting in decomposing from ADP released from damaged vascular endothelial cells, erythrocytes and adhered platelets to AMP (adenosine mono-phosphate) and to exhibit an anti-hemostatic action. To elucidate a vampire behavior of various vampire insects beyond the mosquito, it seems to be essential to analyze the function of their salivary substances.
A number of the salivary substances are predicted to be involved in inhibition of the platelet aggregation, and for example, a platelet aggregation inhibitory activity has been reported for the protein derived from Triatoma infestans (see Patent documents 1 and 2).
Among the proteins derived from the salivary gland in Anopheles stephensi, the protein having a blood coagulation inhibitory activity has been identified, but no protein having the platelet aggregation inhibitory activity has been identified (see Patent Document 3).
In addition, 33 novel proteins have been reported from cloning of cDNA library of the salivary gland in Anopheles stephensi, no protein having the platelet aggregation inhibitory activity is described in the report, and the functions in many of them remains to be unknown (see Non-patent literature 2).
The present inventors previously reported a protein (AAPP) having a GE (Gly Glu)-rich sequence cloned from the salivary gland in Anopheles stephensi (see Non-parent literature 3). The protein has an open reading frame (ORF) of 810 bases and is the protein of 28.5 kDa deduced to be composed of 269 amino acid residues. The protein was thereafter found to be similar to an antigen of 30 kDa (GenBank Accession No. AY226454) disclosed in Non-patent literature 2, but there is no description for actions and functions (activity) of the protein in Non-patent literatures 2 and 3.    [Patent Document 1] JP 2004-121091-A Publication    [Patent document 2] JP 2004-121086-A Publication    [Patent document 3] JP 2003-116573-A Publication    [Non-patent literature 1] Riberio, J. M., J. Exp. Biol., 108, 1-7 (1984))    [Non-patent literature 2] Valenzuela, J. G., et. al., Exploring the salivary gland transcriptome and proteome of the Anopheles stephensi mosquito”, Insect Biochemistry    [Non-patent literature 3] Hiroyuki Watanabe et al., Medical Entomology and Zoology 55 Suppl., pp 41, 19 (2004)